Krypton and xenon are present in air as trace components, 1.14 vppm and 0.086 vppm, respectively, and can be produced in pure form from the cryogenic distillation of air. Both of these elements are less volatile (i.e., have a higher boiling temperature) than oxygen and therefore concentrate in the liquid oxygen sump in a conventional double column air separation unit. Unfortunately, other impurities which are less volatile than oxygen, such as methane, also concentrate in the liquid oxygen sump along with krypton and xenon.
Unfortunately, process streams containing oxygen, methane, krypton and xenon present a safety problem due to the combined presence of methane and oxygen.
Methane and oxygen form flammable mixtures with a lower flammability limit of 5% methane in oxygen. In order to operate safely, the methane concentration in an oxygen stream must not be allowed to reach the lower flammability limit and, in practice, a maximum allowable methane concentration is set that is a fraction of the lower flammability limit. This maximum effectively limits the concentration of the krypton and xenon that are attainable as any further concentration of these products would also result in a methane concentration exceeding the maximum allowed. Therefore, it is desirable to remove methane from the process.
Methane is currently removed from the krypton and xenon concentrate stream using a burner that operates at 800-1000.degree. F. The burning of methane produces two undesirable by-products, water and carbon dioxide, in the process stream. These impurities are typically removed by molecular adsorption. Therefore, the current method of removing methane requires a methane burner, an adsorption system, and several heat exchangers to warm the stream from a cryogenic temperature to the burner temperature and then back to a cryogenic temperature after the adsorption step. Methane removal in this manner also results in some loss of krypton and xenon.
Numerous processes are taught in the background art, among these are the following:
A method of operation of a krypton/xenon column is disclosed in a publication by H. Dauer entitled "New Developments Resulting in Improved Production of Argon, Krypton and Xenon". The relevant portion of the disclosed process is shown in FIG. 1. In the method, liquid oxygen is withdrawn from the bottom of low pressure column of an air separation unit, passed through a hydrocarbon adsorber, and fed to the top of the krypton/xenon column. The hydrocarbon adsorber does not remove methane from the liquid oxygen stream. Liquid in the sump of the krypton/xenon column is reboiled using air from the high pressure column to provide vapor in the krypton/xenon column. Vapor that exits the top of the column contains primarily oxygen with krypton, xenon, and methane. This vapor is added to the gaseous oxygen product stream. Krypton loss in this stream is 11% of the krypton that entered with the liquid oxygen feed. A liquid product stream is recovered from the bottom of the krypton/xenon column that contains a combined krypton and xenon concentration of approximately 0.3% and a methane concentration of 0.5% (the maximum allowable limit). The liquid to vapor ratio (reflux ratio) in the krypton/xenon column is greater than 1.0 at all locations in the column when operated in this manner.
Another process that produces a stream concentrated in krypton and xenon by cryogenic methods is disclosed in U.S. Pat. No. 4,401,448. The process uses two columns to concentrate krypton and xenon in addition to the standard double column air separation unit. In this process, a gaseous oxygen stream is withdrawn from below the first tray of the low pressure column and fed below the first tray of the rare gas stripping column. Reflux for this column is provided by a liquid oxygen stream withdrawn from the low pressure column at a point above where the gaseous oxygen stream was taken. Boilup in the rare gas stripping column is provided by indirect heat exchange with a gaseous nitrogen stream from the high pressure column. Vapor exiting from the top of the rare gas stripping column operates at a reflux ratio of 0.1 to 0.3 (preferred value 0.2). Liquid that is concentrated in krypton, xenon and hydrocarbons is withdrawn from the bottom of rare gas stripping column is fed to the top of the oxygen exchange column. A gaseous nitrogen stream, taken from the high pressure column, is introduced below the first stage of the oxygen exchange column such that the reflux ratio is 0.15 to 0.35 (preferred value 0.24). Boilup in the oxygen exchange column is provided by indirect heat exchange with a gaseous nitrogen stream from the high pressure column. Vapor exiting the top of the oxygen exchange column is recycled to the low pressure column. A liquid product that is concentrated in krypton and xenon is withdrawn from the bottom of the oxygen exchange column.
U.S. Pat. No. 4,401,448 reports results from a computer simulation of the process described above. The liquid product stream withdrawn from the oxygen exchange column contained 1.0% oxygen, 11000 ppm krypton, 900 ppm xenon, and 3200 ppm hydrocarbons with balance being nitrogen. This scheme alleviated two problems associated with prior processes. First, introduction of nitrogen at the bottom of the oxygen exchange column effectively displaces oxygen such that the product stream withdrawn from this column does not contain enough oxygen to form a flammable mixture with hydrocarbons. Second, the process is cryogenic. Krypton recovery was calculated as 72% from data presented in the patent and such a low recovery is undesirable.
Another method of operating a raw krypton column to produce a stream concentrated in krypton and xenon is disclosed in U.S. Pat. No. 4,568,528. A liquid oxygen stream is withdrawn from the low pressure column and introduced to the reboiling zone of the raw krypton column without being passed through a hydrocarbon adsorber. This feed liquid is partially vaporized to produce vapor and a liquid product concentrated in krypton and xenon. The column is refluxed by a liquid having krypton and xenon in lower concentration than the vapor formed in the reboiling zone. This reflux liquid is a stream withdrawn a few trays above the sump of the LP column and contains hydrocarbons that will accumulate in the sump of the raw krypton and limit the krypton/xenon concentration in the product stream. Vapor withdrawn from the top of the column is added to the gaseous oxygen product.
One major disadvantage of this process is the loss of krypton and xenon in a hydrocarbon adsorber which has to be subsequently used to remove hydrocarbons. Since concentration of krypton and xenon in the stream to the hydrocarbon adsorber is higher than that in feed stream, a larger fraction of krypton and xenon is lost as compared to the typical case where a hydrocarbon adsorption unit is used on the feed stream. However, if a hydrocarbon adsorber were to be used on this feed stream then a hydrocarbon adsorption unit will have to be used on the reflux stream which is also contaminated with hydrocarbons. This adds cost and complexity to the process taught in the U.S. Pat. No. 4,568,528.